Add some third_party modules.

backports_abc and singledispatch were made with stubgen, certifi was
written by hand, and ssl_match_hostname was copied from ssl.pyi.

third_party/2and3/backports/ssl_match_hostname.pyi

@@ -0,0 +1,3 @@
+class CertificateError(ValueError): ...
+
+def match_hostname(cert, hostname): ...

third_party/2and3/backports_abc.pyi

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+# Stubs for backports_abc (Python 3.5)
+#
+# NOTE: This dynamically typed stub was automatically generated by stubgen.
+
+from typing import Any
+
+def mk_gen(): ...
+def mk_awaitable(): ...
+def mk_coroutine(): ...
+
+Generator = ... # type: Any
+Awaitable = ... # type: Any
+Coroutine = ... # type: Any
+
+def isawaitable(obj): ...
+
+PATCHED = ... # type: Any
+
+def patch(patch_inspect=True): ...

third_party/2and3/certifi.pyi

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+def where() -> str: ...
+def old_where() -> str: ...

third_party/2and3/singledispatch.pyi

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+# Stubs for singledispatch (Python 3.5)
+#
+# NOTE: This dynamically typed stub was automatically generated by stubgen.
+
+def singledispatch(func): ...